Polymer blends comprising polypropylene are well known in the prior art. Various polymer blends comprising polypropylene are disclosed, for example, in Chundury et al., U.S. Pat. Nos. 5,264,280, 5,321,081, and 5,969,027. Polymer blends comprising polypropylene provide a lower cost alternative to engineering resins such as acrylonitrile butadiene styrene (ABS) polymer resins, and are thus used in a variety of applications including, for example, preparing parts for the automotive industry, for communication devices such as telephones, radios, televisions, cassettes, for power tools, for appliances, for floor care products, for business machines, and in the manufacture of toys and furniture.
Ferro Corporation, the assignee of the present application, markets a variety of polymer blends comprising polypropylene under the OPTUM® trademark. One of the OPTUM® polymer blends sold by Ferro Corporation comprises a calcium carbonate filled blend of polypropylene impact copolymer, styrene-butadiene block copolymer, polystyrene, and styrene-maleic anhydride copolymer. This polymer blend is particularly suitable for use in thermoforming parts for conventional clothes washing appliances. In such applications, this thermoformed polymer blend retains its intended coloration despite being frequently exposed to aqueous detergents that create a neutral to mildly basic pH environment.
Recently, a new type of clothes treating apparatus has been introduced for use by consumers whereby compressed air and an atomized conditioning composition are introduced into the interior region of the apparatus in order to “refresh” one or more articles of clothing hanging therein.
The conditioning or cleaning composition used in these devices creates an environment that is slightly acidic (i.e., a pH of from about 5.0 to about 5.5), which is unusual since most cleaning environments are basic in pH. It was discovered that conventional calcium carbonate filled blends of polypropylene impact copolymer, styrene-butadiene block copolymer, polystyrene, and styrene-maleic anhydride copolymer sold by Ferro Corporation under the OPTUM® mark tend to develop an unsightly pinkish surface discoloration when placed in this acidic environment. The discoloration tends to form after a period of 7-10 days or roughly 20 conditioning cycles. The discoloration does not correlate with widely reported “gas-fading” phenomena. Instead, the discoloration involves the surface deposition of a color-chemical complex borne from prolonged contact of the detergent mist with the thermoformed part. The pinkish surface discoloration could be easily wiped from the surface of the thermoformed interior liner, but it quickly returned upon additional exposure to the acidic detergent mist environment.